The present invention relates to a toner for use in development of electrostatic latent image formed in such processes as electrophotography, electrostatic printing and electrostatic recording.
To take electrophotography as an example of the processes in which the toner of the present invention is to be employed, a electrostatic latent image is formed on a photoconductive photoreceptor through charging and exposure steps and the so formed electrostatic latent image is developed with colored toner particles, with the resulting toner image being subsequently transferred onto an image receiving sheet and fixed with heat or under pressure so as to form a visible image.
A variety of methods have been employed to fix the toner image and the use of hot roll is particularly preferred. In this method, an image-receiving sheet such as paper that carries the toner image is transported in contact with hot roll so as to fix the toner image onto the paper. This fixing method is advantageous not only from the viewpoint of safety but also in terms of energy saving due to reduced heat loss. However, in order to fix the toner image by passage through hot roll, the molten toner particles have to be brought into contact with the surface of the roll and this is prone to cause an undesired phenomenon called "hot offsetting" in which part of the fused toner particles are transferred onto the surface of the roll and thereafter picked up by the image-receiving sheet being fed in the next cycle of process, resulting in a soiled image.
The demand for increasing the operating speed of copying machines or reducing their size is growing today and it is strongly desired to develop a toner the image of which can be fixed at temperatures lower than those which have been feasible in the prior art. If a number of copying cycles are run on a high-speed copier, a substantial amount of the heat generated by the hot rollers is lost to the image-receiving sheet and the supply of heat becomes insufficient to prevent frequent occurrence of poor fixing of the toner image due to the decrease in the temperature of the roll. In small-size copiers, the capacity of the heater for heating the roll must be reduced to realize energy saving and compactness. However, heaters of a smaller capacity require undesirably long time to heat the roll or are unable to supply the necessary heat for enabling continuous copying, which again results in frequent occurrence of poor fixing of the toner image due to the decrease in the temperature of the roll.
Therefore, toners suitable for use in electrophotography must satisfy the following two basic requirements: (1) higher resistance to "hot offsetting"; and (2) effective fixing of the toner image at low temperatures.
The following two techniques have been proposed in an attempt to enable the fixing of toner image at lower temperatures without sacrificing the resistance to hot offsetting:
(1) using as a toner resin a nonlinear polyester that consists of monomers including monomers of trivalence or higher valency and in which the content of such monomers of trivalence or higher valency in all the monomers present is held low and with a carboxylic acid having an alkenyl or alkyl group being introduced in side chains (see Unexamined Published Japanese Patent Application No. 109825/1982, Japanese Patent Application No. 109539/1984 and Unexamined Published Japanese Patent Application No. 7960/1984); and
(2) using as a toner resin a nonlinear polyester that consists of monomers including monomers of trivalence or high valency and in which the content of such monomers of trivalence or higher valency in all the monomers present is held low and with a long-chain aliphatic hydrocarbon group being introduced in the backbone chain (see Japanese Patent Application Nos. 216244/1985 and 217995/1985).
These conventional methods, however, have the disadvantage that when the mixture of starting materials for toner is ground into particles at ambient temperature, the particles have a tendency to form clump matters and subsequent grinding operations cannot be smoothly effected to produce toner particles of a desired size, leading to a lower production rate and a higher cost.
To facilitate the grinding operation, the molecular weight of the toner resin may be lowered tut this could result in a toner that is low in resistance to hot offsetting. An ideal toner should have a good anti-blocking property, namely, it should remain in a stable powder form without agglomerating under use or storage conditions. However, if the molecular weight of a toner resin is lowered, its glass transition point will also decrease to such an extent that the resulting toner particles have a great tendency to agglomerate (i.e., become poor in anti-blocking property).